Researches have been widely made regarding a magnetic-field measuring device for measuring magnetism with high sensitivity on the order of picotesla or nanotesla. For example, there have been proposed magnetic-field measuring devices using a superconducting quantum interference element or device (SQUID) or a magneto-impedance sensor (MI sensor).
Of these magnetic-field measuring devices, the magnetic-field detecting device using the SQUID utilizes a superconducting Josephson effect and a superconducting coil, and has a problem of requirement for a large-scale device for maintaining the magnetic-field detecting device at an extremely low temperature to establish a superconducting state, and equipment for tightly shielding the magnetic-field detecting device from an ambient magnetic field.
On the other hand, the magnetic-field sensing device using the MI sensor is a sensor utilizing a phenomenon that an impedance of the MI element varies, due to a skin effect, depending upon the frequency of an alternating current applied to the sensor, and accordingly has an advantage that the sensor does not require a large-scale device and equipment as required by the above-described magnetic-field detecting device using the SQUID.
However, the MI sensor described above requires electrical connection of conductors to amorphous wires having magnetic anisotropy, for applying the alternating current to the amorphous wires. This electrical connection is made by a method such as soldering and ultrasonic welding, which involves heat application to and vibratory motions of the amorphous wires, causing thermal expansion and contraction and positional displacement of the amorphous wires, and consequent variation of tension of the amorphous wires, giving rise to deterioration of accuracy of detection of the magnetic field by the MI sensor. While the MI sensor is required to have a high degree of uniformity in its quality as a product, it is difficult to control the variation of tension of the amorphous wires due to physical changes of the amorphous wires, namely, their thermal expansion and contraction, and positional displacement. Thus, there was a high risk of difficulty to produce the MI sensor having the required uniformity of quality. FIG. 12 is a view indicating a relationship between the tension of the amorphous wires and the sensitivity of the MI sensor, which was obtained by an experiment conducted by the present inventors. In FIG. 12, the tension of the amorphous wires is taken along the horizontal axis, while the sensitivity of the amorphous wires is taken along the vertical axis. As indicated in FIG. 12, the sensitivity of the amorphous wires, that is, their resolution of detection decreases with an increase of their tension, that is, the sensitivity and the tension of the amorphous wires have a close relationship therebetween. In this respect, the variation of the tension of the amorphous wires attached to the MI sensor is not negligible for assuring the desired performance of the MI sensor as the product.
Where the MI sensor is a gradiometer (gradient magnetic-field detecting device) configured to detect the magnetic field on the basis of a difference between outputs of two sensors, in particular, these two sensors should be produced so as to have the same degree of sensitivity. However, there is a possibility of difficulty to produce the two sensors having the same degree of sensitivity, where the amorphous wires are connected by the soldering or similar method described above.